This invention generally relates to a control circuit for a communications terminal which provides a visual presentation of stored information and more particularly relates to such a control circuit wherein selected information is stored in a random access memory and includes means for continuously editing the memory.
Information display communications terminals are widely used as devices for the reception and transmission of encoded data. Such terminals generally receive and store data from an incoming line or keyboard, with the stored information being displayed upon the screen of a cathode ray tube. A particular feature of such terminals is their ability to store, edit and read out the information in response to selected commands. Heretofore, such devices have generally operated sequentially rather than synchronously. That is, in a sequential device the control circuitry executes various status inquiries and commands individually and in sequence. In this regard, the readout from the memory to the display is sequentially interlaced with other operations which are interrupted during display refresh even though their execution does not command use of the memory. It will be appreciated that such sequential operation considerably reduces the effective speed of the terminal as compared to snychronous operation wherein several terminal operations may be performed simultaneously. The control circuit described in the illustrated embodiment being synchronous allows multiple operations to be performed simultaneously and, so long as the selected operations do not simultaneously command use of the memory, they may be performed concurrently with the display refresh.
A feature provided by some terminals is their ability to display characters having selected attributes. For example, the letter A may be underlined or highlighted to provide it with a distinguishing characteristic, setting the letter apart from the remainder of the displayed text. Prior devices have usually stored such attribute information as a separate work in the memory; each stored character having one or more attribute memory work positions associated therewith. As the number of selected attributes increases, some of which are mutually exclusive, the codes and the number of memory positions necessary to store the codes becomes quite large. Consequently, such an arrangement consumes a considerable amount of memory space thus greatly increasing the cost and complexity of the terminal. With respect to the illustrated embodiment, the attributes are stored as bits in extended code levels of the character code. During readout, the attribute information is converted from its parallel state (extended bit level) and sent from the terminal serially. This arrangement results in a reduction of the memory size and thus the cost and complexity of the terminal are reduced.
With respect to prior terminals, the incoming codes are analyzed to determine the presence of selected machine functions and character attributes. Implementation of this analysis procedure has generally included decoders each of which is adapted to respond to the presence of a predetermined incoming code. A particular difficulty with such an arrangement is its inflexibility in accommodating unique user requirements since the decoders are generally hard-wired. The described embodiment overcomes such inflexibility by analyzing the incoming codes by a comparison system wherein incoming codes are compared against selected codes stored in a preprogrammed memory. A coincidence signal is generated by the comparator indicating that the incoming code is a machine function or character attribute of particular user significance. In this manner, each terminal can conveniently be tailored to the unique requirements of the user by modifying the coding of the preprogrammed memory. This feature permits adaptation of the terminal to user requirements without the necessity of changing the circuitry of the terminal unit.
Thus, the illustrated embodiment provides an improved highly flexible communications terminal capable of manipulating data at increased operational speed. Such advantages are attained without increasing the cost or complexity of the system.